Description At least two
species of ambrosia beetles attack black walnut trees but the most serious is
Xylosandrus germanus (Blandf.). This beetle occurs throughout most of
the northeastern and north-central walnut growing regions.

The adult female beetle is dark brown to
black and about 1/8 inch long. The immature stages are rarely seen because they
occur in tunnels made inside the wood by the adult female. External entrance
holes to these tunnels are about 1/32 inch in diameter, and are sometimes
referred to as pinholes.

Injury Young walnut trees up
to 8 years old are most often attacked.

A Xylosandrus germanus female may
introduce a Fusarium fungus into the tree as she excavates her tunnel into
wood. This fungus causes a cankered area in the wood, usually causing top
dieback and resprouting from the base of the tree. Cankering, however, is not
always apparent. In some plantations, dieback in 1 year due to ambrosia
beetle/Fusarium canker attack has been reported on 30 to 40 percent of the
trees.

Ambrosia beetle attack is usually not
detected until there is profuse sprouting from the base of the trees or until
the trees are dead. Close examination is necessary to locate the tiny pinholes
in the lower stem area or in small, lowhanging branches.

Control Cut and remove dead or
Fusarium-cankerinfected tree tops and branches and burn, if possible.